FIG. 5 is a sectional view of a conventional cylinder head. In FIG. 5, 101 is a hole for attachment of a fuel injection nozzle, 102 is an intake port, 103 is an exhaust port, and 104 is a water jacket. As shown in FIG. 5, a position around the fuel injection nozzle and a position between ports are cooled by cooling water in the water jacket 104. In the cylinder head shown in FIG. 5, the cooling water which flows into the cylinder head from the cylinder block (not shown) is made to flow into one big room (water jacket 104). Consequently, the flow velocity of the cooling water in the water jacket 104 becomes relatively slow (approximately 1 m/sec). It is no problem if the cylinder head shown in FIG. 5 is applied to a relatively low-power engine. However, if the cylinder head shown in FIG. 5 is applied to a relatively high-power engine, it is possible that cooling performance lacks and the cylinder head cracks by temperature rise at the lower surface of the cylinder head.
In order to solve this problem, such a cylinder head is known that the water jacket is divided into two, one of which is in a lower surface side of the cylinder head and another of which is in an upper surface side of the cylinder head. There is an example of this kind of cylinder head that is described in Japanese laid-open Patent Application No. 2000-34950. FIG. 6 is a sectional view of the cylinder head having the water jacket divided into the lower surface side of the cylinder head and the upper surface side of the cylinder head. In FIG. 6, 201 is a hole for attachment of the fuel injection nozzle, 202 is an intake port, 203 is an exhaust port, 204 is a water jacket in the lower surface side of the cylinder head, and 205 is a water jacket in the upper surface side of the cylinder head. In the cylinder head shown in FIG. 6, in view of point that the lower side of the cylinder head is more necessary to be cooled than the upper side of the cylinder head, the cross-sectional area of the water jacket 204 in the lower surface side of the cylinder head is made relatively small, and the cross-sectional area of the water jacket 205 in the upper surface side of the cylinder head is made relatively large. That is, the flow velocity of the cooling water in the water jacket 204 in the lower surface side of the cylinder head is higher than the flow velocity of the cooling water in the water jacket 205 in the upper surface side of the cylinder head. The flow velocity of the cooling water in the water jacket 205 in the lower surface side of the cylinder head becomes approximately 3 m/sec. Therefore, the lower surface side of the cylinder head can be efficiently cooled.
Also in the cylinder head in which the lower surface side can be efficiently cooled, it is known that it is more necessary to cool a side near the fuel injection nozzle than a side apart from the fuel injection nozzle among the circumference of the port. FIG. 7 is a perspective view of an example of the water jacket in the lower surface side of the cylinder head. In FIG. 7, 301 is a fuel injection nozzle part and 302 is a port part. The cooling water flows inside the water jacket in the lower surface side of the cylinder head as shown by arrow in the figure. In the water jacket in the lower surface side of the cylinder head shown in FIG. 7, in view of point that the side near the fuel injection nozzle part 301 is more necessary to be cooled than the side apart from the fuel injection nozzle part 301 among the circumference of the port, the cross sectional area of the water jacket which is in the side apart from the fuel injection nozzle part 301 (part A in the figure) among the circumference of the port part 302 is reduced so that the cooling water is hard to flow in the side apart from the fuel injection nozzle part 301 among the circumference of the port part 302. However, to reduce the cross sectional area of the water jacket means to make the cross sectional area of the sand core used for casting of the cylinder head small. Therefore, it is possible that the core breaks when set before the casting of the cylinder head, and that the core is damaged by the pressure of the molten metal during the casting.
In order to solve this problem, traditionally, such a cylinder head is known that a cooling water pipe as a cooling water passage is located inside of the cylinder head. There is an example of this kind of the cylinder head that is described in Japanese laid-open Patent Application No. 2000-170600. In the cylinder head described in the Japanese laid-open Patent Application No. 2000-170600, a cooling water passage is formed around the exhaust port. In detail, the cooling water passage is formed inside each exhaust valve seat. The cooling water passages which are respectively around a pair of exhaust ports being next to each other are communicated with each other by a pipe. An outlet opening is formed in the pipe, and the cooling water flowing into the pipe from the cooling water passage in the exhaust valve seat is discharged outside of the pipe through the outlet opening. That is, the cooling water in the pipe is discharged toward a position between the pair of adjoining exhaust ports.
However, in the cylinder head described in the Japanese laid-open Patent Application 2000-170600, no outlet opening of the pipe for the cooling water is provided around the fuel injection nozzle, although break or damage of the core when casting of the cylinder head are avoided since the cooling water pipe as a cooling water passage is placed inside the cylinder head. Therefore, in the cylinder head described in the Japanese laid-open Patent Application 2000-170600, a position around the fuel injection nozzle and a position between ports cannot be efficiently, simultaneously cooled.
And also, in the cylinder head described in the Japanese laid-open Patent Application 2000-170600, one cooling water pipe is necessary to cool the position between the pair of adjoining ports. That is, two or more cooling water pipes are necessary to cool each position between each pair of adjoining ports among a plurality of pairs of adjoining ports. That is, in the case that there are two exhaust ports and two intake ports in one cylinder, and that the position between the pair of adjoining exhaust ports and the position between the exhaust port and the intake port which adjoin each other intend to be cooled, a plurality of cooling water pipes are required. Therefore, the structure of the cylinder head will become complicated and the casting process of the cylinder head will become complicated.
Therefore, it is an object of this invention to provide a cylinder head in which the position around the fuel injection nozzle and the position between ports can be efficiently and simultaneously cooled, and the structure of which is simpler and the production of which is easier than the cylinder head described in the Japanese laid-open Patent Application 2000-170600.